In today's Spanish-speaking countries, Diego and Santiago are common as given names; Diego, Santiago and Sandiego are found as surnames. The forms Tiago, Thiago, Diago and Diogo are seen mostly in Portuguese speaking (Lusophone) countries.

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During the medieval era, the names "Sant Yago", "Diago" and "Diego" seem to have coexisted. "Sant Yago" is used, for example, in a letter by James II of Aragon dated 1300: "[...] maestro de la cavalleria de Sant Yago et de la dita orden [...]".[1]

In the Renaissance era, the name was Latinized as Didacus (from Greekδιδαχή (didache) "teaching").[3]

"Diego" as a generic name or term for a Spaniard is documented from around 1615, and "Dago" is used as such still in the 19th century. By the early 20th century, the term "dago" became an ethnic slur chiefly for Italian Americans, besides also for anyone of Spanish or Portuguese descent.[4]

The patronymic for Diego is Díaz in Castillian (used for example by Rodrigo Díaz de Vivar, better known as El Cid) and Diasin Portuguese. Like many patronymics, these have become common surnames in Spanish- and Portuguese-speaking regions. The form Diéguez is much less common; Diegues can be found in Portuguese-speaking countries. de Diego and Diego can also be found as surnames.